Refractive index sensor for analyzing an analyte, and method of fabricating thereof

ABSTRACT

A refractive index sensor is provided for analysing an analyte, the sensor including: a strip waveguide for receiving an input light signal therein and transmitting the light signal, subject to manipulation as it propagates through the strip waveguide, to a detector for analysis with respect to the analyte; and a slot waveguide for sensing the analyte disposed thereon and for receiving a sensing signal, corresponding to said manipulation of the light signal, from the strip waveguide, wherein a grating is formed on a surface of the strip waveguide to enable coupling of the sensing signal from the strip waveguide to the slot waveguide, and the sensor is configured with enhanced sensitivity based on a sensitivity difference between the slot waveguide and the strip waveguide, and/or a group index difference between the slot waveguide and the strip waveguide

CROSS-REFERENCE TO RELATED APPLICATION

The present application is the U.S. National Stage under 35 U.S.C. §371of International Patent Application No. PCTSG2013000432, filed 8 Oct.2013, which claims priority to Singapore Application No. SG 201207483-7,filed 8 Oct. 2012, the disclosures of which are hereby incorporatedherein by reference.

FIELD OF INVENTION

The present invention generally relates to a refractive index sensor foranalyzing an analyte, such as in biochemical analysis, a method offabricating the refractive index sensor, and more particularly, to ahighly sensitive refractive index sensor.

BACKGROUND

Optical refractive index (RI) sensors have been extensively investigatedfor a number of applications and play a prominent role in biochemicalanalysis. Among the existing biochemical RI sensors, those based onintegrated optical waveguides are of interest because of their highsensitivity, small size, and high scale integration. Recently, RIsensors based on certain types of slot waveguides have attractedinterest due to their ability to provide high optical intensity in asubwavelength-scale low refractive index region (slot region). With suchslot waveguides, larger light-analyte interaction in the slot region,and hence higher sensitivity, can be obtained as compared toconventional strip waveguides. Up to now, slot waveguide sensors basedon ring resonator, Mach-Zehnder interferometer, Bragg grating, anddirectional coupler have been reported. The reported slot waveguide ringresonator sensors may exhibit sensitivity of about two times larger(about 212 nm/RIU (refractive index unit)) than that of ring resonatorsensors based on conventional strip waveguides.

However, it would be beneficial to further enhance the sensitivity ofthe RI sensor to improve its analyte detectionmeasurement abilities inorder to detectmeasure biomolecules with very low detection thresholdfor example. In addition, because of the complex nature of mostbiological interactions, it would also be beneficial to provide an RIsensor capable of wavelength multiplexed measurements.

A need therefore exists to provide a reflective index (RI) sensor foranalyzing an analyte which is highly sensitive, and preferably alsocapable of wavelength multiplexed measurements. It is against thisbackground that the present invention has been developed.

SUMMARY

The present invention seeks to overcome, or at least ameliorate, one ormore of the deficiencies of the prior art mentioned above, or to providethe consumer with a useful or commercial choice.

According to a first aspect of the present invention, there is provideda refractive index sensor for analysing an analyte, the sensorcomprising:

-   -   a strip waveguide for receiving an input light signal therein        and transmitting the light signal, subject to manipulation as it        propagates through the strip waveguide, to a detector for        analysis with respect to the analyte; and    -   a slot waveguide for sensing the analyte disposed thereon and        for receiving a sensing signal, corresponding to said        manipulation of the light signal, from the strip waveguide,    -   wherein a grating is formed on a surface of the strip waveguide        to enable coupling of the sensing signal from the strip        waveguide to the slot waveguide, and    -   the sensor is configured with enhanced sensitivity based on a        sensitivity difference between the slot waveguide and the strip        waveguide, and/or a group index difference between the slot        waveguide and the strip waveguide.

Preferably, the sensor further comprises a substrate, wherein the stripwaveguide and the slot waveguide are disposed on the substrate so as tobe spaced apart and substantially parallel to each other.

Preferably, the sensing signal is in the form of a light signal, and thegrating has a grating period configured to couple light signal at aparticular resonant wavelength from the strip waveguide to the slotwaveguide.

Preferably, the slot waveguide has a mode index which is subject tochange based on the analyte disposed thereon, the change in the modeindex results in a shift in the particular resonant wavelength of thelight signal coupled from the strip waveguide to the slot waveguide,thereby enabling analysis of the analyte based on the shift in theparticular resonant wavelength.

Preferably, the sensor is configured such that its sensitivity (S) isdetermined based on the following equation:

$S = {\lambda_{0}\frac{\Delta \; S}{\Delta \; N_{g}}}$

-   -   where, λ₀ is the particular resonant wavelength of the light        signal, ΔS is the sensitivity difference between the slot        waveguide and the strip waveguide, and ΔN_(g) is the group index        difference between the slot waveguide and the strip waveguide.

Preferably, the sensor is configured such that the sensitivitydifference between the slot waveguide and the strip waveguide isincreased and/or the group index difference between the slot waveguideand the strip waveguide is reduced.

Preferably, the strip waveguide is isolated to decrease its sensitivityso as to increase the sensitivity difference between the slot waveguideand the strip waveguide.

Preferably, the strip waveguide is enclosed by an isolation layer madeof SiO₂ to isolate the strip waveguide from the analyte.

In another embodiment, the strip waveguide is enclosed by an isolationlayer made of a polymer material to isolate the strip waveguide from theanalyte.

Preferably, the polymer material has a thermal-optic coefficientselected for compensating a positive or negative temperature dependenceof the sensor so as to reduce the temperature dependence of the sensor.

Preferably, the polymer material is made of WIR30-490 or SU-8.

Preferably, a width of the strip waveguide is configured to be about 600nm to about 1000 nm.

Preferably, one or more parameters of the slot waveguide are configuredto increase its sensitivity so as to increase the sensitivity differencebetween the slot waveguide and the strip waveguide, said one or moreparameters include a width of the slot waveguide and/or a width of a gapof the slot waveguide.

Preferably, the slot waveguide is configured such that the width of theslot waveguide is increased and/or the width of the gap is reduced.

Preferably, the width of the slot waveguide is in the range of about 350nm to about 550 nm, and the width of the gap is in the range of about 50nm to about 300 nm.

Preferably, a plurality of gratings, spaced apart from each other, isformed on the surface of the strip waveguide, each grating having adifferent grating period configured for coupling a sensing signal at arespective resonance wavelength to the slot waveguide, thereby enablingwavelength multiplexed measurement.

Preferably, the slot waveguide and/or the strip waveguide are made ofsilicon nitride (Si₃N₄).

According to a second aspect of the present invention, there is provideda method of fabricating a refractive index sensor for analysing ananalyte, the method comprising:

-   -   forming a strip waveguide for receiving an input light signal        therein and transmitting the light signal, subject to        manipulation as it propagates through the strip waveguide, to a        detector for analysis with respect to the analyte; and    -   forming a slot waveguide for sensing the analyte disposed        thereon and for receiving a sensing signal, corresponding to        said manipulation of the light signal, from the strip waveguide,    -   wherein the method further comprises forming a grating on a        surface of the strip waveguide to enable coupling of the sensing        signal from the strip waveguide to the slot waveguide, and    -   configuring the sensor with enhanced sensitivity based on a        sensitivity difference between the slot waveguide and the strip        waveguide, and/or a group index difference between the slot        waveguide and the strip waveguide.

According to a third aspect of the present invention, there is provideda refractive index sensor device for analysing an analyte, the sensordevice comprising:

-   -   a refractive index sensor comprising a strip waveguide and a        slot waveguide;    -   a light source for outputting a light signal to the strip        waveguide; and    -   a detector for receiving the light signal from the strip        waveguide for analysis with respect to the analyte;    -   wherein the strip waveguide is configured to receive the light        signal therein from the light source and transmit the light        signal, subject to manipulation as it propagates through the        strip waveguide, to the detector for analysis,    -   the slot waveguide is configured for sensing the analyte        disposed thereon and for receiving a sensing signal,        corresponding to said manipulation of the light signal, from the        strip waveguide,    -   a grating is formed on a surface of the strip waveguide to        enable coupling of the sensing signal from the strip waveguide        to the slot waveguide, and    -   the refractive index sensor is configured with enhanced        sensitivity based on a sensitivity difference between the slot        waveguide and the strip waveguide, and/or a group index        difference between the slot waveguide and the strip waveguide.

Preferably, the refractive index sensor device further comprises apolarization controller for receiving the light signal from the lightsource, and outputting a TE polarized light signal to the stripwaveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be better understood andreadily apparent to one of ordinary skill in the art from the followingwritten description, by way of example only, and in conjunction with thedrawings, in which:

FIG. 1A depicts a schematic perspective view of the RI sensor accordingto an exemplary embodiment of the present invention;

FIG. 1B depicts a schematic cross-sectional view of the RI sensor;

FIG. 2 illustrates the coupling of the sensing signal from the stripwaveguide to the slot waveguide and shows the transmission spectra atthe outputs of the two waveguides;

FIGS. 3A and 3B depict the field distributions for the strip waveguideand slot waveguide, respectively, of the sensor having exemplaryparameters.

FIG. 3C depicts a graph showing the dependences of the mode indexN_(eff) of the two waveguides and the mode index difference ΔN_(eff)between the two waveguides 104, 112 as a function of the wavelength 2;

FIG. 3D depicts a graph showing the dependences of the mode indexN_(eff) of the two waveguides and the mode index difference ΔN_(eff)between the two waveguides 104, 112 as a function of the externalrefractive index n_(ex);

FIG. 4 depicts a schematic cross-sectional view of a sensor according toanother embodiment of the present invention whereby the strip waveguideis isolated.

FIGS. 5A to 5C show the sensitivity of the two sensors (i.e., with andwithout isolation) against the width W_(s) of the slot waveguide, thewidth g of the gap 124 of the slot waveguide, and the rib height t;

FIG. 6 depicts the variation of the coupling coefficient andcorresponding grating lengths required for achieving κL=π/2 as afunction of the etch depth for three exemplary values of separationdistance s;

FIGS. 7A and 7B show the transmission spectra for the two sensors (i.e.,without and with isolation), respectively, at different values ofexternal refractive index n_(ex);

FIG. 7C illustrates the linear dependence of the resonance wavelength λ₀on the external refractive index n_(ex);

FIG. 8A depicts a schematic cross-sectional view of a sensor accordingto yet another embodiment of the present invention for reducingtemperature dependence of the sensor.

FIG. 8B shows the temperature dependence of the resonance wavelength forthe sensors of FIGS. 1B, 4 and 8 at different values of strip waveguidewidth;

FIG. 9 depicts a sensor according to an embodiment of the presentinvention configured for enabling wavelength multiplexed measurement;

FIG. 10 depicts a refractive index sensor device for analysing ananalyst according to an embodiment of the present invention; and

FIG. 11 depicts a flow chart generally illustrating a method offabricating a refractive index sensor for analysing an analyte accordingto an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention seek to provide a highly sensitivereflective index (RI) sensor for analyzing an analyte, and a method offabricating the reflective index sensor. Details of the RI sensoraccording to exemplary embodiments of the present invention will now bedescribed.

FIG. 1A depicts a schematic perspective view and FIG. 1B depicts aschematic cross-sectional view of the RI sensor 100 according to anexemplary embodiment of the present invention. As illustrated, thesensor 100 for analysing an analyte, such as in biochemical analysis,includes a strip waveguide 104 for receiving an input light signaltherein and transmitting the light signal, subject to manipulation as itpropagates through the strip waveguide 104, to a detector for analysiswith respect to the analyte 108. For example, the input light signal maybe a broad-band light from a broad-band optical light source 1004 andthe detector may be an optical spectrum analyser (OSA) 1008 as shown inFIG. 10. The input light signal is received at one end (e.g., an inputend) 105 from the light source 1004 and output at another end (e.g., anoutput end) 106 of the strip waveguide 104. The sensor 100 furtherincludes a slot waveguide 112 for sensing the analyte 108 disposedthereon and for receiving a sensing signal, corresponding to theabove-mentioned manipulation of the light signal, from the stripwaveguide 104. With this configuration, the slot waveguide 112 functionsas a sensing waveguide while the strip waveguide 104 functions as asignal waveguide for the optical signal.

In practice, the analyte 108 may be disposed in a sensing area 113generally indicated by the dashed enclosure in FIG. 1A, and preferablyon a section 115 of the slot waveguide 112 opposing a grating section116 (described below) of the strip waveguide 104. In the exemplaryembodiment of FIGS. 1A and 1B, the analyte 108 is shown in FIG. 1B to bedisposed on or covers both the strip waveguide 104 and the slotwaveguide 112 in the sensing area 113. However, it is neither necessarynor essential for the analyte to be disposed on the strip waveguide 104since it is the slot waveguide 112 that functions as the sensingwaveguide.

The sensor 100 preferably further includes a substrate 114, and thestrip waveguide 104 and the slot waveguide 112 are disposed on thesubstrate 114 so as to be side by side. More specifically, the stripwaveguide 104 and the slot waveguide 112 are spaced apart by a distances (see FIG. 1B) and substantially parallel to each other.

In the exemplary embodiment, a grating 116 is formed on a surface 118 ofthe strip waveguide 104 to enable coupling of the sensing signal fromthe strip waveguide 104 to the slot waveguide 112. This coupling of thesensing signal is schematically illustrated in FIG. 2. The sensingsignal is in the form of a light signal, and the grating 116 has agrating period Λ configured to couple light signal at a particularresonant wavelength λ₀ from the strip waveguide 104 to the slotwaveguide 112. Furthermore, the sensor 100 is configured with enhancedsensitivity based on a sensitivity difference ΔS between the slotwaveguide 112 and the strip waveguide 104, and/or a group indexdifference ΔN_(g) between the slot waveguide 112 and the strip waveguide104. This will be described in further detail later below.

The slot waveguide 112 comprises two parallel strips (or rails) 122having a high refractive index separated by a low-index region (i.e.,slot or gap) 124. As shown in FIG. 1B, the strip waveguide 104 has awidth W, the slot waveguide 112 has a width W_(s), and the gap 124 has awidth g. The strip waveguide 104 and the slot waveguide 112 may have thesame height h and rib height t. For clarity and illustration purpose,exemplary embodiments are hereinafter described with the strip waveguide104 and slot waveguide 112 made of silicon nitride (Si₃N₄), and thesubstrate 114 made of SiO₂. It will be appreciated to a person skilledin the art that the waveguides 104, 112 and the substrate 114 are notlimited to such materials and other suitable/appropriate materials arewithin the scope of the present invention. For example, the stripwaveguide 104 and the slot waveguide 112 may instead be made of Silicon(Si) or high refractive index polymers. If different materials are used,it will be appreciated to a person skilled in the art that theabove-mentioned design parameters of the sensor (e.g., g, W, W_(s), s,etc.) 100 may also have to be adjusted accordingly. In FIG. 1B, therefractive index of Si₃N₄, SiO₂, and the external medium (analyte) 108are denoted as n_(Si3N4), n_(SiO2) and n_(ex), respectively.

As shown in FIG. 1A, the grating 116 is formed on a top surface 118 ofthe strip waveguide 104. Without the grating 116, no light couplingwould occur between the strip waveguide 104 and the slot waveguide 112since they are not synchronous in phase (i.e. they have differentpropagation constants). In the exemplary embodiment, the phasesynchronism is achieved with the grating 116 formed on the top surface118 of the strip waveguide 104. In this regard, with a predeterminedgrating period Λ, a light power transfer from the strip waveguide 104 tothe slot waveguide 112 is obtained at a particular wavelength (resonancewavelength λ₀) satisfying the phase-matching condition:

λ₀=(N _(eff) ^(strip) −N _(eff) ^(slot))Λ=ΔN _(eff)Λ,  (1)

where N_(eff) ^(strip) and N_(eff) ^(slot) are the mode indices of thestrip waveguide 104 and slot waveguide 112, respectively. Therefore,according to Equation (1), the grating coupler 116 iswavelength-selective. As illustrated in FIG. 2, when a broad-band light204 is launched into the strip waveguide 104, light at the resonancewavelength λ₀ is coupled to the slot waveguide 112. This produces aband-rejection spectrum 208 (center wavelength at λ₀) in the launchingstrip waveguide 104 while a band-pass spectrum 212 (center wavelength atλ₀) in the neighbouring slot waveguide 112. FIG. 2 shows thetransmission spectra 208, 212 at the outputs of the two waveguides 104,112 as a result of the grating coupler 116 having a predeterminedgrating period Λ. This illustrates the manipulation of the input lightsignal 204 as it propagates through the strip waveguide 104, and thesensing signal coupled to the slot waveguide 112 corresponding to such amanipulation of the input light signal 204.

As shown in FIGS. 1A and 1B, the slot waveguide 112 functions as asensing waveguide while the strip waveguide 104 functions as a signalwaveguide for the optical signal guiding and detection. As therefractive index of the analyte (n_(ex)) 108 changes, the mode indicesof the slot waveguide 112 and strip waveguide 104 change accordingly.This therefore results in a change in the difference ΔN_(eff) betweenthe mode indices of the strip waveguide 104 (N_(eff) ^(strip)) and theslot waveguide 112 (N_(eff) ^(slot)), which in turn leads to a shift inthe resonance wavelength λ₀ of the light coupled from the stripwaveguide 104 to the slot waveguide 112. The resonance wavelength λ₀ ofthe light signal 206 output from the strip waveguide 104 will thereforeshift correspondingly and can be detected by the detector 1008.Accordingly, the analyte 108 may be analysed based on this shift in theresonant wavelength λ₀. Accordingly, the sensitivity S of the sensor 100may be defined as the degree of such a shift in the resonant wavelengthλ₀ in response to the analyte 108.

With the configuration of the sensor 100 as described in the exampleembodiment, the RI sensitivity S of the sensor 100 may be defined as:

$\begin{matrix}{{S = {\frac{\lambda_{0}}{n_{ex}} = {{\frac{\lambda_{0}}{\left( {N_{g}^{strip} - N_{g}^{slot}} \right)}\left( {\frac{\partial N_{eff}^{strip}}{\partial n_{ex}} - \frac{\partial N_{eff}^{slot}}{\partial n_{ex}}} \right)} = {\lambda_{0}\frac{\Delta \; S}{\Delta \; N_{g}}}}}}{where}} & (2) \\{{N_{g}^{strip} = {N_{eff}^{strip} - {\lambda \frac{N_{eff}^{strip}}{\lambda}}}},{N_{g}^{slot} = {N_{eff}^{slot} - {\lambda \frac{N_{eff}^{slot}}{\lambda}}}},{{\Delta \; N_{g}} = {N_{g}^{strip} - N_{g}^{slot}}},} & (3) \\{{S^{strip} = \frac{\partial N_{eff}^{strip}}{\partial n_{ex}}},{S^{slot} = \frac{\partial N_{eff}^{slot}}{\partial n_{ex}}},{{\Delta \; S} = {S^{strip} - {S^{slot}.}}}} & (4)\end{matrix}$

According to Equations (2) to (4), it is found that the sensitivity S ofthe sensor 100 described in the example embodiment is proportional tothe sensitivity difference ΔS between the strip waveguide 104 and theslot waveguide 112. With this configuration, due to the high intensityfield distribution in the slot region 124 of the slot waveguide 112, thesensitivity S^(slot) of the slot waveguide 112 is much larger than thatS^(strip) of the strip waveguide 104, therefore resulting in a largersensitivity difference ΔS. In addition, as can be seen from Equation(2), the sensitivity S of the sensor 100 is also inversely proportionalto the group index difference ΔN_(g) between the strip waveguide 104 andthe slot waveguide 112. The group index difference ΔN_(g) according tothe example embodiment is configured to have a small value. Accordingly,the sensor 100 can be configured with greatly enhanced sensitivity basedon the above factors (i.e., the sensitivity difference ΔS and/or thegroup index difference ΔN_(g)). In contrast, the sensitivity ofconventional single slot or strip waveguide based sensors is typicallyonly inversely proportional to the group index N_(g) (i.e., not thegroup index difference) which generally has a much larger value (e.g.,about 2 to 4). Therefore, such conventional sensors have a much smallersensitivity.

For illustrate purpose only and without limitation, a sensor 100according to the exemplary embodiment having the following exemplaryparameters will now be examined, including an exemplary calculation ofthe sensitivity S of the sensor 100. In particular, the exemplaryparameters are: n_(Si3N4)=2.0, n_(ex)=1.333, n_(SiO2)=1.444, h=400 nm,g=200 nm, s=1 μm, W=1 μm, W_(s)=450 nm, and t=0 nm. In this example,only the TE polarization is considered.

FIGS. 3A and 3B show the field distributions for the strip waveguide 104and slot waveguide 112, respectively, of the sensor 100 with theabove-mentioned exemplary parameters. It can be observed from FIG. 3Bthat the light intensity inside the nanoscale (200 nm) low refractiveindex slot region 124 of the slot waveguide 112 is very strong. FIGS. 3Cand 3D illustrate the graphs showing the dependences of the mode indexN_(eff) of the two waveguides 104, 112 and the mode index differenceΔN_(eff) between the two waveguides 104, 112 as a function of thewavelength 2 (i.e., FIG. 3C) and external refractive index n_(ex) (i.e.,FIG. 3D). Therefore, the group index N_(g) ^(strip), N_(g) ^(slot) andthe sensitivity S^(strip), S^(slot) of each waveguide 104, 112 may becalculated from the graphs shown in FIGS. 3C and 3D. Thecharacteristics/properties of the sensor 100 calculated, including thesensitivity S of the sensor 100, are shown in Table 1 below.

TABLE 1 Properties of the exemplary sensor shown in FIG. 1A (having theabove- mentioned exemplary parameters) Sensitivity (S) dλ₀/dn_(ex) N_(g)^(strip) N_(g) ^(slot) ΔN_(g) S_(strip) S_(slot) ΔS (nm/RIU) 2.04791.7902 0.2577 0.1793 0.4222 −0.2429 −1461.0

As shown in Table 1, the sensitivity S of the sensor 100 in this examplecan advantageously be as large as −1461 nm/RIU. It will be understoodthat the negative sign implies that the resonance wavelength λ₀decreases as the external refractive index increases (n_(ex)).Significantly, this sensitivity S value is about 20 times larger thanthat of a conventional strip waveguide ring resonator sensor and about 7times larger than a conventional slot waveguide ring resonator sensor.

Therefore, the configuration of the sensor 100 as described in theexemplary embodiment (i.e., based on the sensitivity difference ΔSbetween the slot waveguide and the strip waveguide, and/or the groupindex difference ΔN_(g) between the slot waveguide 112 and the stripwaveguide 104) has been demonstrated to advantageously result in ahighly sensitive refractive index sensor 100.

According to further embodiments of the present invention, thesensitivity (S) of the sensor 100 can be further enhanced byconfiguring/adjusting the sensitivity difference ΔS between the slotwaveguide 112 and the strip waveguide 104, and/or the group indexdifference ΔN_(g) between the slot waveguide 112 and the strip waveguide104. This is evident from Equation (2) described above which shows that

$S = {\lambda_{0}{\frac{\Delta \; S}{\Delta \; N_{g}}.}}$

Therefore, by configuring the sensor 100 such that the sensitivitydifference ΔS between the slot waveguide 112 and the strip waveguide 104is increased and/or the group index difference ΔN_(g) between the slotwaveguide 112 and the strip waveguide 104 is reduced, the sensitivity Sof the sensor 100 may be further enhanced.

In exemplary embodiments, to increase the sensitivity difference ΔSbetween the slot waveguide 112 and the strip waveguide 104, thesensitivity S^(strip) of the strip waveguide 104 is decreased and/or thesensitivity S^(slot) of the slot waveguide 112 is increased. In apreferred embodiment, the sensitivity S^(strip) of the strip waveguide104 is decreased by isolating the strip waveguide 104 as illustrated inFIG. 4. The sensor 400 depicted in FIG. 4 is the same as the sensor 100depicted in FIG. 1B, except that the strip waveguide 104 of the sensor400 is enclosed by an insolation layer 404 to isolate it from theexternal analyte 108. It should be noted that the same or similarreference numerals in FIGS. 1A and 1B are applied to the same or similarparts/components throughout the drawings (including FIG. 4), and thedescription of the same or similar parts/components will be omitted orsimplified. A preferred or suitable material for the isolation layer 404is silicon dioxide (SiO₂). It will be appreciated to a person skilled inthe art that the isolation layer 404 is not limited to an SiO₂ isolationlayer and other suitable/appropriate materials are within the scope ofthe present invention. For example, in a preferred embodiment describedwith reference to FIG. 8A later below, the isolation layer is made of alow index material such as a polymer material (e.g., an epoxy resin suchas SU-8 and various types of photoresist).

For illustrate purpose only and without limitation, thecharacteristics/properties of the sensor 400 having the same exemplaryparameters as described hereinbefore and with the SiO₂ isolation layerdepicted in FIG. 4 are calculated and shown in Table 2 below.

TABLE 2 Properties of the exemplary sensor shown in FIG. 4 (having theabove- mentioned exemplary parameters). Sensitivity (S) dλ₀/dn_(ex)N_(g) ^(strip) N_(g) ^(slot) ΔN_(g) S_(strip) S_(slot) ΔS (nm/RIU)2.0248 1.7858 0.2390 0.0026 0.4193 −0.4167 −2702.4

By comparing the sensitivity S_(strip) of the strip waveguide 104 inTables 1 and 2, it can be clearly seen that S_(strip) is significantlyreduced after the SiO₂ isolation. As a result, the sensitivitydifference ΔS between the slot waveguide 112 and the strip waveguide 104increased significantly, which therefore leads to a correspondinglylarge increase in the sensitivity of the sensor 400. As shown in Table2, the sensitivity of the sensor 400 calculated is about −2702.4 nm/RIUwhich is almost 2 times larger than that the sensor 100 describedhereinbefore without the strip waveguide being isolated. Thisdemonstrates an effective way to decrease the sensitivity of the stripwaveguide 104 in the interest of increasing the sensitivity of thesensor 400.

As mentioned above, the sensitivity difference ΔS between the slotwaveguide 112 and the strip waveguide 104 can also be increased byincreasing the sensitivity S^(slot) of the slot waveguide 112. In anembodiment, this can be achieved by configuring one or more parametersof the slot waveguide 112. To demonstrate this, FIGS. 5A, 5B, and 5Cshow the sensitivity S of the sensors 100, 400 against the width W_(s)(in the range of about 350 nm to 550 nm) of the slot waveguide 112, thewidth g (in the range of about 100 nm to 300 nm) of the gap 124 of theslot waveguide 112, and the rib height t (in the range of 0 to 100 nm).From FIGS. 5A and 5B, for both sensors 100, 400, it can be clearly seenthat the sensitivity S increases as the width W_(s) of the slotwaveguide 112 and/or the width g of the gap 124 of the slot waveguide112 decreases. Therefore, it has been demonstrated that byconfiguring/adjusting the parameters of the slot waveguide 112, thesensitivity S of the sensors 100, 400 can be as high as about 1700nm/RIU and 3500 nm/RIU, respectively. On the other hand, from FIG. 5C,the sensitivity S of the sensors 100, 400 has only be found to weaklydepend on the rib height t.

In a preferred embodiment, the width W_(s) of the slot waveguide 112 isin the range of about 350 nm to 550 nm, and more preferably 450 nm to550 nm, the width g of the gap 124 of the slot waveguide 112 is in therange of about 100 nm to 300 nm, and more preferably 100 nm to 200 nm,and the rib height t is in the range of about 0 nm to 100 nm.

The coupling coefficient and the transmission spectrum of the grating116 will now be described. The transmission spectrum at the output ofthe strip waveguide 104 can be obtained based on the following equation:

$\begin{matrix}{{{T(\lambda)} = {1 - {\frac{\kappa^{2}}{\kappa^{2} + {\delta^{2}/4}}\sin^{2}\sqrt{\kappa^{2} + {\delta^{2}/4}}L}}}{where}} & (5) \\{\delta = {{\frac{2\pi}{\lambda}\Delta \; N_{eff}} - \frac{2\pi}{\Lambda}}} & (6)\end{matrix}$

In the above equations, L is the grating length and κ is the couplingcoefficient used to characterize the strength of the grating and isobtained with:

$\begin{matrix}{{\kappa = {\frac{2\left( {n_{{Si}_{3}N_{4}}^{2} - n_{ex}^{2}} \right)}{\lambda \; c\; \mu_{0}}{\int{\int_{A}{{{\overset{\rightharpoonup}{e}}_{strip} \cdot {\overset{\rightharpoonup}{e}}_{slot}^{*}}{A}}}}}},} & (7)\end{matrix}$

where c and μ₀ are the speed of light in free space and the vacuumpermeability. {right arrow over (e)}_(strip) and {right arrow over(e)}_(slot) are the normalized fields of strip waveguide 104 and slotwaveguide 112, respectively. A denotes the grating area. According toEquation (5), when κL=π/2 and δ=0, 100% coupling occurs at the resonancewavelength λ₀.

According to an embodiment, the grating strength (i.e., couplingcoefficient) can be controlled by configuring the grating etch depth onthe top surface 118 of the strip waveguide 104. To demonstrate this,FIG. 6 shows the variation of the coupling coefficient as a function ofthe etch depth for three values of separation distance s. It can beobserved that the coupling coefficient increases as the etch depthincreases. In addition, smaller separation distance results in largercoupling coefficient at the same etch depth. The corresponding gratinglengths required for achieving κL=π/2 are also shown in FIG. 6. Forexample, with L=1500 μm, the coupling coefficient required for achievinga maximum contrast is given by κ=π/2L=1.047×10³ m⁻¹, which according toFIG. 6, requires an etch depth of about 28 nm (about 7% of the thicknessof the waveguide height).

FIGS. 7A and 7B show the transmission spectra for the two sensors 100,400 (i.e., without and with SiO₂ isolation), respectively, at differentvalues of external refractive index n_(ex). The grating lengths for bothsensors 100, 400 are set to 2000 μm. In both Figures, the resonancewavelength λ₀ shifted to shorter wavelength as the external refractiveindex increases. By comparing FIGS. 7A and 7B, it can be observed thatthe sensor 400 with SiO₂ isolation is more sensitive than the sensor 100without SiO₂ isolation. In FIG. 7C, the dependence of the resonancewavelength λ₀ on the external refractive index n_(ex) are shown to belinear.

In a further embodiment, a sensor 800 as depicted in FIG. 8A isconfigured with reduced or minimal temperature dependence. The sensor800 depicted in FIG. 8A is the same as the sensor 100 depicted in FIG.1B or the sensor 400 depicted in FIG. 4 except that the strip waveguide104 of the sensor 400 is isolated or enclosed by a polymer layer 804. Itshould be noted that the same or similar reference numerals are appliedto the same or similar parts/components throughout the drawings, and thedescription of the same or similar parts/components will be omitted orsimplified.

In the embodiment, the temperature dependence of the resonancewavelength λ₀ is calculated as follow:

$\begin{matrix}{{\frac{\lambda_{0}}{T} = {{\frac{\lambda_{0}}{\Delta \; N_{g}}\left( {{\frac{{\partial\Delta}\; N_{eff}}{\partial n_{{SiO}_{2}}}C_{{SiO}_{2}}} + {\frac{{\partial\Delta}\; N_{eff}}{\partial n_{ex}}C_{ex}} + {\frac{{\partial\Delta}\; N_{eff}}{\partial n_{cover}}C_{cover}} + {\frac{{\partial\Delta}\; N_{eff}}{\partial n_{{Si}_{3}N_{4}}}C_{{Si}_{3}N_{4}}}} \right)} = {\frac{\lambda_{0}}{\Delta \; N_{g}}F_{t}}}},} & (8)\end{matrix}$

where C_(SiO2), C_(ex), C_(cover), C_(Si3N4) are thermal-opticcoefficients (TOC) for the SiO₂ substrate 114, external analyte 108,polymer cover layer 804 for isolation, and Si₃N₄, respectively. In thisexample, it was found that the sensor 400 has a positive temperaturedependence (i.e., resonance wavelength λ₀ shifts to longer wavelength astemperature increases). To address this positive temperature dependence,the embodiment provides a polymer cover 804 with a negative TOC as anisolation layer instead of the SiO₂ layer 404 as disclosed in theembodiment of FIG. 4 to compensate for the temperature dependence of thesensor 400. It was found that F_(t) in Eq. (8) can be a small valueclose to zero when a polymer material with an appropriate negative TOCis used. That is, selecting a polymer material having a TOC which cansubstantially compensate the positive temperature dependence of thesensor such that the net temperature dependence is minimal orsubstantially reduced, and vice versa. Therefore, the temperaturedependence of the sensor 800 can advantageously be significantly reducedor substantially eliminated.

For illustration purpose, an experiment was conducted for each of thesensors 100, 400, 800 with the following parameters: C_(SiO2)=1.0×10⁻⁵/°C., C_(ex)=−8.0×10⁻⁵/° C. (i.e., for water), C_(cover)=−1.8×10⁻⁴/° C.,C_(Si3N4)=4.0×10⁻⁵/° C., n_(cover)=1.49 (the TOC and the refractiveindex are typical values of polymers). FIG. 8B shows the temperaturedependence of the resonance wavelength for each of the three sensors100, 400, 800 at different values of strip waveguide width (from 900 nmto 1000 nm). It can be seen that the sensors without and with SiO₂isolation (i.e., 100, 400) have a positive temperature dependence ofabout 200 pm/° C. and about 310 pm/° C., respectively. On the otherhand, when a polymer isolation layer 804 is used, the temperaturedependence is significantly reduced to be less than 20 pm/° C. for alarge range of strip waveguide width (i.e., large tolerance). Inaddition, the temperature dependence can be further reduced tosubstantially zero by choosing a strip waveguide 104 having a width ofabout 950 nm. This can be derived from FIG. 8B since the crossing pointof the dashed line (corresponding to zero temperature sensitivity) andthe curve with polymer isolation is located when W is around 950 nm.Therefore by applying a polymer isolation layer 804, not only is thesensitivity enhanced, but the temperature dependence is also greatlyreduced. In general, any polymer material that has an appropriate TOC issuitable, and preferably the polymer material is patternable. Forexample and without limitation, the polymer material may be WIR30-490 orSU-8.

According to another embodiment, the sensor 900 is configured so as toenable wavelength multiplexed measurement. As mentioned hereinbefore,the grating 116 is intrinsically wavelength-selective. Therefore, thesensor 100, 400, 800 can be extended to a configuration suitable forwavelength multiplexed measurement. FIG. 9 illustrates a top schematicview of a sensor 900 capable of wavelength multiplexed sensing wherefour cascaded gratings 116 a, 116 b, 116 c, and 116 d with differentperiods Λ₁, Λ₂, Λ₃, and Λ₄ (corresponding to different resonancewavelengths) are formed along the top surface 118 of the strip waveguide104. Each grating 116 generates a sensing signal at the respectiveresonance wavelength λ₁, λ₂, λ₃, and λ₄, therefore, a spectrum with fourpeaks can be observed at the output of the strip waveguide 104. Thewavelength of each peak can then be monitored or detected for wavelengthmultiplexed measurement.

Accordingly, embodiments of the present invention provide a highlysensitive refractive index sensor, such as in biochemical analysis,based on grating assisted co-directional light coupling between a stripwaveguide 104 and a slot waveguide 112. The sensor has a highsensitivity and can be further enhanced by isolating the strip waveguide104 and/or optimizing the slot waveguide parameters. With a polymerisolation layer, the sensor can further achieve minimal or significantlyreduced temperature dependence. In addition, the sensor can beconfigured to have wavelength multiplexed measurement capability due tothe intrinsic wavelength-selective property of the gratings. The sensorsdisclosed in the exemplary embodiments have a wide range of applicationssuch as but not limited to clinical applications where multiplexeddetection of biomolecules with low detection limit is desirable. Forexample, other applications can be in the fields of environmentalmonitoring, and food safety and drug screening, such as a gas sensor forlow-concentration explosive gas detection.

FIG. 10 depicts a refractive index sensor device 1000 for analysing ananalyst incorporating the refractive index sensor 100, 400 or 800described hereinbefore according to exemplary embodiments of the presentinvention. In particular, the refractive index sensor device 1000includes a refractive index sensor 100, 400, or 800 comprising a stripwaveguide 104 and a slot waveguide 112, a light source (e.g., abroadband optical source) 1004 for outputting a light signal to thestrip waveguide 104, and a detector (e.g., an optical spectrum analyzer(OSA)) 1008 for receiving the light signal from the strip waveguide 104for analysis (preferably wavelength shift analysis) with respect to theanalyte 108. The strip waveguide 104 is configured to receive the lightsignal therein from the light source 1004 and transmit the light signal,subject to manipulation as it propagates through the strip waveguide, tothe detector 1008 for analysis. The slot waveguide 112 is configured forsensing the analyte disposed thereon and for receiving a sensing signal,corresponding to the above-mentioned manipulation of the light signal,from the strip waveguide 104. Furthermore, a grating 116 is formed on asurface of the strip waveguide 104 to enable coupling of the sensingsignal from the strip waveguide 104 to the slot waveguide 112. Inparticular, the refractive index sensor 100, 400, or 800 is configuredwith enhanced sensitivity based on a sensitivity difference between theslot waveguide 112 and the strip waveguide 104, and/or a group indexdifference between the slot waveguide 112 and the strip waveguide 104.

Preferably, the refractive index sensor device 1000 further includes apolarization controller 1006 for receiving the light signal from thelight source 1004, and outputting a TE polarized light signal to thestrip waveguide.

FIG. 11 depicts a flow chart generally illustrating a method 1100 offabricating a refractive index sensor for analysing an analyte. Themethod 1100 includes a step 1102 of forming a strip waveguide forreceiving an input light signal therein and transmitting the lightsignal, subject to manipulation as it propagates through the stripwaveguide 104, to a detector (e.g., an optical spectrum analyser) 1008for analysis with respect to the analyte 108, and a step 1104 of forminga slot waveguide 112 for sensing the analyte 108 disposed thereon andfor receiving a sensing signal, corresponding to the above-mentionedmanipulation of the light signal, from the strip waveguide 104. Themethod 1100 further includes a step 1106 of forming a grating on asurface of the strip waveguide to enable coupling of the sensing signalfrom the strip waveguide to the slot waveguide. It will be appreciatedto a person skilled in the art that the above-described steps may beperformed in any order and are not limited to the order presented.Furthermore, the above steps are not intended to be construed tonecessitate individual steps and may be combined as one fabrication stepwhere appropriate without deviating from the scope of the presentinvention.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

1. A refractive index sensor for analysing an analyte, the sensorcomprising: a strip waveguide for receiving an input light signaltherein and transmitting the light signal, subject to manipulation as itpropagates through the strip waveguide, to a detector for analysis withrespect to the analyte; and a slot waveguide for sensing the analytedisposed thereon and for receiving a sensing signal, corresponding tosaid manipulation of the light signal, from the strip waveguide, whereina grating is formed on a surface of the strip waveguide to enablecoupling of the sensing signal from the strip waveguide to the slotwaveguide, and the sensor is configured with enhanced sensitivity basedon a sensitivity difference between the slot waveguide and the stripwaveguide, and/or a group index difference between the slot waveguideand the strip waveguide.
 2. The sensor according to claim 1, furthercomprises a substrate, wherein the strip waveguide and the slotwaveguide are disposed on the substrate so as to be spaced apart andsubstantially parallel to each other.
 3. The sensor according to claim1, wherein the sensing signal is in the form of a light signal, and thegrating has a grating period configured to couple light signal at aparticular resonant wavelength from the strip waveguide to the slotwaveguide.
 4. The sensor according to claim 3, wherein the slotwaveguide has a mode index which is subject to change based on theanalyte disposed thereon, the change in the mode index results in ashift in the particular resonant wavelength of the light signal coupledfrom the strip waveguide to the slot waveguide, thereby enablinganalysis of the analyte based on the shift in the particular resonantwavelength.
 5. The sensor according to claim 3, wherein the sensor isconfigured such that its sensitivity (S) is determined based on thefollowing equation:$S = {\lambda_{0}\frac{\Delta \; S}{\Delta \; N_{g}}}$ where, λ₀ isthe particular resonant wavelength of the light signal, ΔS is thesensitivity difference between the slot waveguide and the stripwaveguide, and ΔN_(g) is the group index difference between the slotwaveguide and the strip waveguide.
 6. The sensor according to claim 1,wherein the sensor is configured such that the sensitivity differencebetween the slot waveguide and the strip waveguide is increased and/orthe group index difference between the slot waveguide and the stripwaveguide is reduced.
 7. The sensor according to claim 1, wherein thestrip waveguide is isolated to decrease its sensitivity so as toincrease the sensitivity difference between the slot waveguide and thestrip waveguide.
 8. The sensor according to claim 7, wherein the stripwaveguide is enclosed by an isolation layer made of SiO₂ to isolate thestrip waveguide from the analyte.
 9. The sensor according to claim 7,wherein the strip waveguide is enclosed by an isolation layer made of apolymer material to isolate the strip waveguide from the analyte. 10.The sensor according to claim 9, wherein the polymer material has athermal-optic coefficient selected for compensating a positive ornegative temperature dependence of the sensor so as to reduce thetemperature dependence of the sensor.
 11. The sensor according to claim10, wherein the polymer material is made of WIR30-490 or SU-8.
 12. Thesensor according to claim 1, wherein a width of the strip waveguide isconfigured to be about 600 nm to about 1000 nm.
 13. The sensor accordingto claim 1, wherein one or more parameters of the slot waveguide areconfigured to increase its sensitivity so as to increase the sensitivitydifference between the slot waveguide and the strip waveguide, said oneor more parameters include a width of the slot waveguide and/or a widthof a gap of the slot waveguide.
 14. The sensor according to claim 13,wherein the slot waveguide is configured such that the width of the slotwaveguide is increased and/or the width of the gap is reduced.
 15. Thesensor according to claim 13, wherein the width of the slot waveguide isin the range of about 350 nm to about 550 nm, and the width of the gapis in the range of about 50 nm to about 300 nm.
 16. The sensor accordingto claim 1, wherein a plurality of gratings, spaced apart from eachother, is formed on the surface of the strip waveguide, each gratinghaving a different grating period configured for coupling a sensingsignal at a respective resonance wavelength to the slot waveguide,thereby enabling wavelength multiplexed measurement.
 17. The sensoraccording to claim 1, wherein the slot waveguide and/or the stripwaveguide are made of silicon nitride (Si₃N₄).
 18. A method offabricating a refractive index sensor for analysing an analyte, themethod comprising: forming a strip waveguide for receiving an inputlight signal therein and transmitting the light signal, subject tomanipulation as it propagates through the strip waveguide, to a detectorfor analysis with respect to the analyte; and forming a slot waveguidefor sensing the analyte disposed thereon and for receiving a sensingsignal, corresponding to said manipulation of the light signal, from thestrip waveguide, wherein the method further comprises forming a gratingon a surface of the strip waveguide to enable coupling of the sensingsignal from the strip waveguide to the slot waveguide, and configuringthe sensor with enhanced sensitivity based on a sensitivity differencebetween the slot waveguide and the strip waveguide, and/or a group indexdifference between the slot waveguide and the strip waveguide.
 19. Arefractive index sensor device for analysing an analyte, the sensordevice comprising: a refractive index sensor comprising a stripwaveguide and a slot waveguide; a light source for outputting a lightsignal to the strip waveguide; and a detector for receiving the lightsignal from the strip waveguide for analysis with respect to theanalyte; wherein the strip waveguide is configured to receive the lightsignal therein from the light source and transmit the light signal,subject to manipulation as it propagates through the strip waveguide, tothe detector for analysis, the slot waveguide is configured for sensingthe analyte disposed thereon and for receiving a sensing signal,corresponding to said manipulation of the light signal, from the stripwaveguide, a grating is formed on a surface of the strip waveguide toenable coupling of the sensing signal from the strip waveguide to theslot waveguide, and the refractive index sensor is configured withenhanced sensitivity based on a sensitivity difference between the slotwaveguide and the strip waveguide, and/or a group index differencebetween the slot waveguide and the strip waveguide.
 20. The refractiveindex sensor device according to claim 19, further comprising apolarization controller for receiving the light signal from the lightsource, and outputting a TE polarized light signal to the stripwaveguide.